In this thesis I investigate aesthetic preference for picture books among children aged 3–6 as well as criteria for preference among parents. Through my design project: illustration of a picture book, I aim to put my research on preference in the context of illustration and design. By using the design project as an applied method for research I have sought to answer how one as an illustrator can apply knowledge regarding preferences of the target groups when illustrating for picture books. I have primarily based my research within the field of cognitive developmental psychology, with primary focus on stage theory, and the field of literature. I also performed small scale studies on preference as well as a web survey. The results regarding childrens preferences confirm previous studies. It shows that children base preference mainly on subject matter and colour. They also show preference for saturated colours and pictures with less realism. The parents tended to like what their children like regarding colour. Message, conversation and humour were found to be significant, but not generalizable criteria for preference among parents. These findings were summarized and used as guide lines during the process of illustrating a picture book.